This application claims priority to an application entitled xe2x80x9cApparatus and Method for Performing Handoff in a Mobile Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Dec. 30, 1999 and assigned Serial No. 99-67038, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an apparatus and method for performing a handoff considering a moving speed of a mobile station.
2. Description of the Related Art
In general, a mobile communication system divides a radio network into small areas, called xe2x80x9ccellsxe2x80x9d, in order to efficiently manage the frequency resources, and a mobile station (MS) is provided with a service from a base station (BS) in a cell to which it belongs. Meanwhile, when the mobile station moves from a serving cell to another cell, a handoff is performed on the mobile station to maintain the service.
As stated above, in the mobile communication system, the handoff is performed in order to maintain communication even when the mobile station moves between the cells. There have been proposed two handoff methods: one is a soft handoff and another is a hard handoff. The soft handoff performs the service through the channel allocated by a handoff target (or neighbor) base station and the channel allocated by the serving base station, and releases the channel from the serving base station when a channel quality of the serving base station drops below a predetermined threshold. On the contrary, the hard handoff releases the channel from the serving base station and allocates to the mobile station a channel from a neighbor base station, when the channel quality of the serving base station drops below a predetermined threshold during a call.
FIG. 1 shows the overall configuration of a mobile communication system. Referring to FIG. 1, mobile stations 111-11n communicate with their associated base stations 121-12n, which are controlled by one base station controller (BSC) 131. Base station controllers 131-13n are controlled by one mobile switching center (MSC) 141. If the mobile station 111 attempts communication with a wire subscriber 161 of a public switched telephone network (PSTN) 151 through the radio network (comprised of BS, BSC and MSC), the mobile switching center 141 connects the mobile station 111 to the wire subscriber 161 through the PSTN 151 to establish a traffic channel.
FIG. 2 shows a change in strength of the signals received from a serving base station BS1 and a neighbor base station BS2, when the mobile station moves from a position xe2x80x98axe2x80x99 to a position xe2x80x98bxe2x80x99 in an overlapped area (OL) where a service area CELL1 of the serving base station BS1 overlaps with a service area CELL2 of the neighbor base station BS2.
A conventional soft handoff operation will be described below with reference to FIG. 2. In the following description, it will be assumed that the mobile station being provided with the service from the serving base station BS1 moves toward the cell area CELL2 of the neighbor base station BS2.
The soft handoff occurs under the control of the base station controller, when the mobile station is located in the overlapped area OL where the service area CELL1 of the BS1 overlaps with the service area CELL2 of the BS2. That is, upon detecting received strength, from the BS2, corresponding to a predetermined handoff add threshold T_add while moving toward the BS2, the mobile station being serviced by the BS1 sends the detected strength to the base station controller. Upon receipt of the detected strength, the base station controller determines whether the BS2 is available. If the BS2 is available, the base station controller allocates a channel between the BS2 and the mobile station. When the channel is allocated to the BS2, the BS1 and the BS2 provide the mobile station with the service through their allocated channels. Meanwhile, as the mobile station moves toward the BS2, the mobile station will detect the signal strength received from the BS1 dropping below a predetermined handoff drop threshold T_drop. In this case, the mobile station drops the present serving base station BS1 and is provided with the service from only the BS2. Here, the mobile station drops the BS1 after a lapse of a guard time T_Tdrop from the time point when the signal strength received from the BS1 drops below the handoff drop threshold T_drop. That is, the mobile station drops the BS1 only when the signal strength dropped below T_drop is maintained for the guard time T_Tdrop, so as to prevent occurrence of the handoff even when the signal strength is instantaneously dropped below the handoff drop threshold T_drop.
However, this method equally performs the handoff on the mobile station moving at high speed and the mobile station moving at low speed, without considering the moving speed of the mobile station. In this case, the mobile station moving at high speed passes the handoff area very fast, so that the call may be dropped. On the contrary, the mobile station moving at low speed may perform an unnecessary handoff.
It is therefore an object of the present invention to provide a handoff apparatus and method in which a mobile station determines its moving direction and moving speed by measuring a change in a PN phase of a specific pilot and performs a handoff depending on the determined information.
It is another object of the present invention to provide an apparatus and method for increasing a handoff start threshold when the mobile station slowly approaches a handoff target base station, so as to perform a handoff at a later time.
It is further another object of the present invention to provide an apparatus and method for decreasing the handoff start threshold when the mobile station quickly approaches the handoff target base station, so as to perform the handoff at an earlier time.
It is yet another object of the present invention to provide a handoff apparatus and method for adaptively adjusting a received strength threshold of a pilot signal in consideration of a moving speed of the mobile station in a mobile communication system.
To achieve the above and other objects, there is provided an apparatus for performing a handoff in a mobile communication system. A searcher calculates energy of every PN phase in a given search window and reports a detected PN phase to a controller. The controller then determines a moving direction and a moving speed of a mobile station depending on the detected PN phase and a previously detected PN phase, and adaptively determines a pilot received strength threshold used to perform a handoff depending on the determined moving direction and moving speed of the mobile station.